The present invention generally relates to devices for controlling the movement of elevators, and more specifically to an elevator safety device.
Archimedes is said to have built the first elevator in about 236 BC. In 1852, Elisha Otis introduced the safety elevator, which prevented the fall of the cab if the cable broke. The design of the Otis safety elevator is somewhat similar to one type still used today: a governor device engages knurled roller(s), locking the elevator to its guides should the elevator descend at excessive speed. Otis demonstrated his safety device at the New York exposition in the Crystal Palace in a dramatic, death-defying presentation in 1854, and the first such passenger elevator was installed at 488 Broadway in New York City on Mar. 23, 1857. The first electric elevator was built by Werner von Siemens in 1880 in Germany. The safety and speed of electric elevators were significantly enhanced by Frank Sprague, who added floor control, automatic elevators, acceleration control of cars, and safeties. His elevator ran faster and with larger loads than hydraulic or steam elevators, and 584 electric elevators were installed before Sprague sold his company to the Otis Elevator Company in 1895. Sprague also developed the idea and technology for multiple elevators in a single shaft. In 1882, when hydraulic power was a well-established technology, a company later named the London Hydraulic Power Company was formed, and constructed a network of high-pressure mains on both sides of the Thames which, ultimately, extended to 184 miles and powered some 8,000 machines, predominantly elevators (lifts) and cranes. In 1874, J. W. Meaker patented a method which permitted elevator doors to open and close safely (U.S. Pat. No. 147,853). In 1887, Alexander Miles of Duluth, Minn. patented an elevator with automatic doors that would close off the elevator shaft.
Today, elevators are typically powered by electric motors that either drive traction cables or counterweight systems like a hoist, or pump hydraulic fluid to raise a cylindrical piston like a jack. A modern-day elevator consists of a cab (also called a “cage” or “car”) mounted on a platform within an enclosed space called a shaft or “hoistway.” In the past, elevator drive mechanisms were powered by steam and water hydraulic pistons or by hand. In a “traction” elevator, cars are pulled up by means of steel ropes rolled over a deeply-grooved pulley or “sheave.” The weight of the car is balanced by a counterweight. Sometimes, two elevators are built so that their cars always move synchronously in opposite directions, and function as each other's counterweight. The friction between the ropes and the pulley furnishes the traction which gives this type of elevator its name.
Hydraulic elevators use hydraulic power to pressurize an above-ground or in-ground piston to raise and lower the car. Roped hydraulics use a combination of both ropes and hydraulic power to raise and lower cars. Recent innovations include permanent magnet motors, machine room-less rail (MRL) mounted gearless machines, and microprocessor controls.
MRL elevators are designed so that most of the components fit within the shaft containing the elevator car, while a small cabinet houses the elevator controller. Other than the machinery in the hoistway, the equipment is similar to a normal traction elevator. Benefits of an MRL elevator include: more usable space; uses less energy (70-80% less than hydraulic elevators); uses no oil; all components are above-ground similar to roped hydraulic-type elevators (removing the environmental concern created by hydraulic cylinders used on direct hydraulic-type elevators stored underground); slightly lower cost than other elevators; and can operate at faster speeds than hydraulics but not normal traction units. Detriments of MRL elevators include: equipment can be harder to service and maintain, and no code has been approved for the installation of residential MRL elevator equipment.
The technology used in new installations depends on a variety of factors. Hydraulic elevators are cheaper, but installing cylinders greater than a certain length becomes impractical for very-high lift hoistways. For buildings over about 7-stories, traction elevators must be employed instead. Hydraulic elevators are usually slower than traction elevators.
Elevators are a candidate for mass customization. There are economies to be made from mass production of the components, but each building comes with its own requirements, such as a different number of floors, varying well dimensions, and differing usage patterns.
A more detailed discussion of conventional elevator design now follows, which will be helpful in understanding the invention and its advantages.
Traction Elevators
Geared traction elevators are driven by AC or DC electric motors. Geared elevators use worm gears to control mechanical movement of elevator cars by “rolling” steel hoist ropes over a drive sheave which is attached to a gearbox driven by a high-speed motor. These machines are generally the best option for basement or overhead traction use for speeds up to 500 feet per minute (3 m/s). Historically, AC motors were used for single- or double-speed elevator machines on the grounds of cost and lower-usage applications where car speed and passenger comfort were less of an issue. For higher-speed, larger-capacity elevators, the need for infinitely-variable speed control over the traction machine becomes an issue, and DC machines powered by an AC/DC motor generator have been the preferred solution. The MG set also typically powered the relay controller of the elevator, which has the added advantage of electrically isolating the elevators from the rest of a building's electrical system, thus eliminating the transient power spikes in the building's electrical supply caused by the motors starting and stopping (causing lighting to dim every time the elevators are used for example), as well as interference to other electrical equipment caused by the arcing of the relay contactors in the control system.
The widespread availability of variable frequency AC drives has allowed AC motors to be used universally, bringing with it the advantages of the older motor-generator, DC-based systems, without the penalties in terms of efficiency and complexity. Older MG-based installations are gradually being replaced in older buildings due to their poor energy efficiency.
Gearless traction elevators are low-speed (low-RPM), high-torque electric motors powered either by AC or DC. In this case, the drive sheave is directly attached to the end of the motor. Gearless traction elevators can reach speeds of up to 2,000 feet per minute (10 m/s), or even higher. A brake is mounted between the motor and drive sheave (or gearbox) to hold the elevator stationary at a floor. This brake is usually an external drum type and is actuated by spring force and held open electrically; a power failure will cause the brake to engage and prevent the elevator from falling.
With either geared or gearless traction elevators, cables are attached to a hitch plate on top of the cab or may be “underslung” below a cab, and then looped over the drive sheave to a counterweight attached to the opposite end of the cables, which reduces the amount of power needed to move the cab. The counterweight is located in the hoistway and rides a separate railway system; as the car goes up, the counterweight goes down, and vice versa. This action is powered by the traction machine which is directed by the controller, which is typically a relay logic or computerized device that directs starting, acceleration, deceleration and stopping of the elevator cab. The weight of the counterweight is typically equal to the weight of the elevator cab plus 40-50% of the capacity of the elevator. The grooves in the drive sheave are specially designed to prevent the cables from slipping. “Traction” is provided to the ropes by the grip of the grooves in the sheave. As the ropes age and the traction grooves wear, some traction is lost and the ropes must be replaced and the sheave repaired or replaced. Sheave and rope wear may be significantly reduced by ensuring that all ropes have equal tension, thus sharing the load evenly. Rope tension equalization may be achieved using a rope tension gauge, and is a simple way to extend the lifetime of the sheaves and ropes.
Elevators with more than 100 feet (30 m) of travel have a system called compensation. This is a separate set of cables or a chain attached to the bottom of the counterweight and the bottom of the elevator cab. This makes it easier to control the elevator, as it compensates for the differing weight of cable between the hoist and the cab. If the elevator cab is at the top of the hoist-way, there is a short length of hoist cable above the car and a long length of compensating cable below the car and vice versa for the counterweight. If the compensation system uses cables, there will be an additional sheave in the pit below the elevator, to guide the cables. If the compensation system uses chains, the chain is guided by a bar mounted between the counterweight railway lines.
Hydraulic Elevators
Conventional hydraulic elevators use an underground cylinder, and are quite common for low level buildings with 2-5 floors (sometimes but seldom up to 6-8 floors), and have speeds of up to 200 feet per minute (1 m/s). Holeless hydraulic elevators were developed in the 1970s, and use a pair of above-ground cylinders, which makes it practical for environmentally or cost-sensitive buildings with 2, 3, or 4 floors. Roped hydraulic elevators use both above-ground cylinders and a rope system, allowing the elevator to travel further than the piston has to move.
The low mechanical complexity of hydraulic elevators in comparison to traction elevators makes them ideal for low rise, low traffic installations. They are less energy efficient: as the pump works against gravity to push the car and its passengers upwards; this energy is lost when the car descends on its own weight. The high current draw of the pump when starting up also places higher demands on a building's electrical system. There are also environmental concerns should the lifting cylinder leak fluid into the ground.
The modern generation of low cost, machine room-less traction elevators made possible by advances in miniaturization of the traction motor and control systems challenges the supremacy of the hydraulic elevator in their traditional market niche. However, hydraulically-controlled systems generally allow for greater control than traction elevators and make it possible to safety move the elevator to a controlled door zone for safe release form the elevator.
Controlling Elevators
Early elevators had no automatic landing positioning. Elevators were operated by elevator operators using a motor controller. The controller was contained within about a 1′ by 2′ cylindrical container operated using a projecting handle. This allowed some control over the energy supplied to the motor (located at the top of the elevator shaft or beside the bottom of the elevator shaft) and so enabled the elevator to be accurately positioned, if the operator was sufficiently skilled. More typically, the operator would have to “jog” the control to get the elevator reasonably close to the landing point and then direct the outgoing and incoming passengers to “watch their step.” Some older freight elevators are controlled by switches operated by pulling on adjacent ropes. Safety interlocks ensure that the inner and outer doors are closed before the elevator is allowed to move. Most older, manually-controlled elevators have been retrofitted with automatic or semi-automatic controls.
Automatic elevators began to appear as early as the 1930s. Their development was hastened by striking elevator operators taking advantage of large cities such as New York and Chicago dependent on skyscrapers (and therefore their elevators). These electromechanical systems used relay logic circuits of increasing complexity to control the speed, position and door operation of an elevator or bank of elevators. Relay-controlled elevator systems remained common until the 1980s, when they were gradually replaced with solid-state microprocessor-based controls, which are now the industry standard.
General Elevator Controls
A typical modern passenger elevator includes an overload sensor which prevents the elevator from moving until any excess load has been removed, and which may trigger a voice prompt or buzzer alarm and/or trigger a “full car” indicator, indicating the car's inability to accept more passengers until some are unloaded. In addition to such comforts as electric fans and air conditioning units, modern elevators also typically include a control panel with call buttons to choose a floor, some of which may include key switches (to control access). In some elevators, certain floors may be inaccessible unless a security card is swiped or a password is entered, or both.
A set of doors may be kept locked on each floor to prevent unintentional access into the elevator shaft. The door may be unlocked and opened by a machine sitting on the roof of the car, which may also drive the doors that travel with the car. Door controls may be provided to immediately close or reopen the doors, although the button to close them immediately is often disabled during normal operations, especially on more recent elevators. Objects in the path of the moving doors will either be detected by sensors or physically activate a switch that reopens the doors. Otherwise, the doors will close after a preset time. Some elevators are configured to remain open at the floor until they are required to move again.
Elevators in high traffic buildings often have a “nudge” function, which will close the doors at a reduced speed, and sound a buzzer if the “door open” button is being deliberately held down, or if the door sensors have been blocked for too long a time. A stop switch (not allowed under British regulations) may also be provided to halt the elevator while in motion, and to hold an elevator open while freight is loaded. Keeping an elevator stopped for too long may set off an alarm. Unless local codes require otherwise, this will most likely be a key switch. An alarm button or switch, or a telephone, may also be provided, which passengers can use to warn the premises manager that they have been trapped in the elevator. Other controls may be provided as well.
Elevator Safety
Statistically speaking, cable-borne elevators are extremely safe. Of the 20 to 30 elevator-related deaths each year, most of them are maintenance-related, such as technicians leaning too far into the shaft or getting caught between moving parts, and most of the rest are attributed to other kinds of accidents, such as people stepping blindly through doors that open into empty shafts or being strangled by scarves caught in the doors. While it is possible, though unlikely, for an elevator's cable to snap, all elevators in the modern era have been fitted with several safety devices which prevent the elevator from simply free-falling and crashing. An elevator cab is typically borne by six or eight hoist cables, each of which is capable on its own of supporting the full load of the elevator plus twenty-five percent more weight. In addition, there is a device which detects whether the elevator is descending faster than its maximum designed speed; if this happens, a device called a “governor” causes copper brake shoes to clamp down along the vertical rails in the shaft, stopping the elevator quickly, but not so abruptly as to cause injury. In addition, a hydraulic or mechanical buffer may be installed at the bottom of the shaft to somewhat cushion any impact.
Early hydraulic elevators built prior to a code change in 1972 were subject to possible catastrophic failure. The code had previously required only single-bottom hydraulic cylinders. In the event of a cylinder breach, an uncontrolled fall of the elevator might result. Because it is impossible to verify the system completely without a pressurized casing, it is necessary to remove the piston to inspect it. The cost of removing the piston is such that it makes no economic sense to re-install the old cylinder; therefore it is necessary to replace the cylinder and install a new piston. Another solution to protect against a cylinder blowout is to install a “life jacket.” This is a device which, in the event of an excessive downward speed, clamps onto the cylinder and stops the car. A device known as a rupture valve is often attached to the hydraulic inlet/outlet of the piston and can be adjusted for a maximum flow rate. If a pipe or hose were to rupture, the flow rate of the rupture valve will surpass a set limit and mechanically stop the outlet flow of hydraulic fluid, thus stopping the piston and the car in the down direction.
ASME Codes
The American Society of Mechanical Engineers (ASME) has a specific section of Safety Code (ASME A17.1, Section 5.3) which addresses Residential Elevators. This section allows for different parameters to alleviate design complexity based on the limited use of a residential elevator by a specific user or user group. Section 5.3 of the ASME A17.1 Safety Code is for Private Residence Elevators, which does not include multi-family dwellings. Some types of residential elevators do not use a traditional elevator shaft, machine room, and elevator hoistway. This allows an elevator to be installed where a traditional elevator may not fit, and simplifies installation. The ASME Board first approved machine-room-less (MRL) systems in a revision of the ASME A17.1 in 2007. MRL elevators have been available commercially since the mid 1990s; however cost and overall size prevented their adoption to the residential elevator market until around 2010. (As of 2006, all states except Kansas, Mississippi, North Dakota, and South Dakota have adopted some version of ASME codes, though not necessarily the most recent.) Passenger elevators must also conform to many ancillary building codes including the Local or State building code, National Fire Protection Association standards for Electrical, Fire Sprinklers and Fire Alarms, Plumbing codes, and HVAC codes.
Existing Safety Concerns
Safety concerns for elevators remain. For example, while modern codes have regulated unintended and uncontrolled movement of elevators in the “down” direction, as to the about 300,000 traction elevators in the U.S., as well as the about 600,000 hydraulic elevators in the U.S., unintended and uncontrolled elevator movement is not regulated in the “up” direction. In addition, there is a significant void in the regulation of hydraulic elevators, as the codes currently require mechanical safety devices, such as a rope gripper on traction elevators, to control downward unintended and/or uncontrolled elevator movement, but such mechanical safety devices are often not useable with hydraulic elevators, and no such codes govern such movement for hydraulic elevators. As a result, hydraulic elevators often include no safety device, or improper safety devices, for unintended or uncontrolled movement, such as may be caused by catastrophic failure of the hydraulic system. (As one example, piston grippers have been installed in lieu of a piston replacement required by code for hydraulic jack assemblies manufactured prior to 1972. The piston gripper has certain limitations based on physical space required for the unit. Activation of the roper gripper in most cases causes passengers entrapment.)
Additionally, all current safety systems, used with either traction-type or hydraulic-type elevators, create an abrupt stop, and also entrap the passengers within the elevator until maintenance or emergency personnel can move the car to an adjacent floor and pry open the doors, which can take hours.
Accordingly, it is one of the objects of the present invention to provide a safety system for unintended and/or uncontrolled elevator movement that is safer and less cumbersome to install than available such systems, and that may be provided either on new elevators or retrofit on older elevators.
It is another object of the present invention to provide a safety system which, when activated, will safely and smoothly guide an elevator car, under control, to the next available floor and immediately release its passengers.
It is still another object of the present invention to provide a safety system which may be easily installed on hydraulic elevators, as well as on geared and gearless traction elevators.
It is yet another object of the present invention to provide such a safety system which is both cost-efficient to manufacture and easy to install or retrofit.
Further objects of the present invention may be realized by embodiments which: monitor elevator safety circuits, the encoder, the door circuit and the brake or valve circuits; and provide battery back-up in case of power failure.